1. Technical Field
The present invention relates to applying coatings to substrates and, in particular, to using a spin application to dry a coating on the substrate. Still more particularly, the present invention provides a method, apparatus, and program for using surface features to provide a uniform thickness of coatings on a substrate.
2. Description of the Related Art
Typical holographic exposures require a very thin coating of photoresist (e.g. about 800 angstroms) on a surface to record submicron lines and spaces. On planar surfaces, a coating of a desired thickness is typically obtained by adjusting the viscosity of the photoresist and spinning the sample at moderate speeds (e.g. 3500 rpm), with the photoresist freshly applied, for a set time (e.g. about 30 seconds). As the spinning of the sample begins, centrifugal force moves the excess liquid photoresist to the edge and off the sample, evaporating some of the solvents in the photoresist and leaving a coating of the solids, contained in the photoresist on the surface of the sample. This coating process is typically completed with a soft-bake to evaporate the remaining solvents.
In the case of non-planar samples, a combination of centrifugal force, surface tension forces, and features rising out of the plane of the sample surface results in a non-uniform coating across the sample. Generally, at the onset of spinning, photoresist moves outward from the center of the sample and encounters a feature rising above the plane of the surface. The photoresist accumulates at the rise of the feature, leaving a thick coating at the rise of the feature and then leaving a thin coating after the fall of the feature. Rotational forces and surface tensions that are present on the surface complicate the non-uniformity created by this process.
A photoresist may be used to form a feature or component on the surface of the sample. In some applications, a non-uniform photoresist coating may result in an unreliable performance of the resulting component. More particularly, fabrication of a diffraction, refraction, or reflection grating structures for Light Amplification by the Simulated Emission of Radiation (laser) may be adversely affected by an uneven coating. To make an accurate and effective grating, a uniform photoresist coating must be formed in the area of interest. Non-uniform photoresist coating may also affect other fabrication processes, such as grayscale masks.
Therefore, it would be advantageous to provide coating thickness control to ensure uniform distribution of photoresist using a spin coating process.